1. Field of the Invention
The present invention relates to an optical pickup actuator, and more particularly, to an optical pickup actuator having an improved and slim structure which can secure an efficient tracking.
2. Description of the Related Art
In general, optical pickups are installed in optical recording and/or reproducing apparatuses to record and/or reproduce information on and/or from a recording medium, such as an optical disc, and move in a radial direction of the optical disc without contacting the optical disc.
The optical pickups require an actuator which moves an objective lens in a tracking direction, a focusing direction, and/or a tilting direction to spot a laser beam emitted from a light source on a correct portion of the optical disc. Here, a tracking direction movement indicates an adjustment of the objective lens in the radial direction of the optical disc to form a light spot in a center of a track.
A general optical pickup actuator includes a bobbin which is movably installed on a base, suspensions which support the bobbin to allow the bobbin to move above the base, and magnetic circuits which are installed in the bobbin and the base.
The optical pickup actuator basically carries out tracking and focusing movements, i.e., a biaxial movement. It is a general tendency that the recording and/or reproducing apparatuses are miniaturized and become lightweight thereof while using high recording density media.
For the use of the high recording density media, the optical pickup actuator requires a triaxial or quadriaxial movement including a tilting movement in addition to the existing biaxial movement. Recently, for the use of the high recording density media, as a numerical aperture (NA) of the objective lens has been made larger, and a wavelength of the laser beam of the light source has been shortened, a tilting margin of the optical pickup actuator has been decreased. Thus, a triaxial or quadriaxial movement optical pickup actuator, which is able to perform the tilting movement as well as the existing biaxial movements, is required. The triaxial movement includes the focusing movement, the tracking movement, and a radial tilting movement, while the quadriaxial movement includes a tangential tilting movement in addition to the above motions. The biaixial, triaxial, or quadriaxial movement of the optical pickup actuator depends on a configuration of the magnetic circuits of the actuator.
Also, the optical pickup actuator needs to be reduced in height to be miniaturized.
FIG. 1 is a view of an example of a conventional magnetic circuit used in an optical pickup actuator. The use of the magnetic circuit shown in FIG. 1 enables the optical pickup actuator to perform a triaxial movement.
Referring to FIG. 1, the conventional magnetic circuit includes a magnet 1 which is divided into four polarized surfaces to be properly distributed into N poles and S poles, first and second focusing coils 3 and 5, and first and second tracking coils 7 and 9.
The first and second focusing coils 3 and 5, and the first and second tracking coils 7 and 9 are installed on sides of a moving unit of the optical pickup actuator, i.e., a bobbin. The magnet 1 is installed on a base to face the first and second focusing coils 3 and 5 and the first and second tracking coils 7 and 9.
As shown in FIG. 1, on a y–z coordinate plane, first through fourth polarizations 1a, 1b, 1c, and 1c of the magnet 1, respectively corresponding to first through fourth quadrants, are an N pole, an S pole, an N pole, and an S pole, respectively. The first focusing coil 3 ranges over the first and fourth polarizations 1a and 1d, and the second focusing coil 5 ranges over the second and third polarizations 1b and 1c. The first tracking coil 7 ranges over the first and second polarizations 1a and 1b, and the second tracking coil 9 ranges over the third and fourth polarizations 1c and 1d. 
Using the conventional magnetic circuit having the above-described structure, a moving unit of the optical pickup actuator can move in a focusing direction, a tracking direction, and a tilting direction.
When currents flow in the first and second focusing coils 3 and 5 in counterclockwise and clockwise directions, respectively, a force acts in the first and second focusing coils 3 and 5 in a +focusing direction (z-axis direction). When directions of the currents flowing in the first and second focusing coils 3 and 5 are changed to opposite directions, respectively, the force acts in the first and second focusing coils 3 and 5 in a −focusing direction (-z-axis direction). Thus, an objective lens mounted in the moving unit of the optical pickup unit can move in the focusing direction.
When the currents are supplied to the first and second focusing coils 3 and 5 in the same direction (clockwise direction), the force acts in the first focusing coil 3 in the +focusing direction (z-axis direction), and the force acts in the second focusing coil 5 in the −focusing direction (-z-axis direction). Also, when the directions of the currents applied to the first and second focusing coils 3 and 5 are respectively changed into opposite directions, the force acts in the first focusing coil 3 in the −focusing direction (-z-axis direction), and the force acts in the second focusing coil 5 in the +focusing direction (z-axis direction). Thus, the moving unit of the optical pickup actuator can move in the tilting direction, e.g., in a radial tilting direction, to adjust a tilt of the objective lens mounted in the moving unit.
When the currents flow in the first and second tracking coils 7 and 9 in clockwise and counterclockwise directions, the force acts in the first and second tracking coils 7 and 9 in a left direction (-y-axis direction). When the directions of the currents flowing in the first and second tracking coils 7 and 9 are changed to opposite directions, respectively, the force acts in the first and second tracking coils 7 and 9 in a right direction (y-axis direction). As a result, since the moving unit of the optical pickup actuator can move in the tracking direction, the moving unit can control the objective lens mounted therein so as to correctly follow a track.
Accordingly, if a pair of magnetic circuits having the above-described structure are installed on corresponding ones of two sides of the moving unit of the optical pickup actuator, the moving unit can move in the focusing, tracking, and radial tilting directions, i.e., in triaxial directions.
However, due to an arrangement of the polarizations 1a through 1d of the magnet 1 for the triaxial movement, the conventional magnetic circuit having the above-described structure should have the first and second tracking coils 7 and 9, that are disposed in the focusing direction, and the first and second tracking coils 7 and 9 have to be spaced apart from each other. Thus, since effective coil lengths of portions (marked with slanting lines in FIG. 1) of the first and second tracking coils 7 and 9 contributing to the tracking direction movement are short, the conventional magnetic circuit has difficulty in becoming slim (reducing a height thereof) when securing an efficient tracking. Also, when the conventional magnetic circuit becomes slim, it has a structural drawback in considerably deteriorating a tracking efficiency.